Move on_goal to std

Stream: std4

[Jannis Limperg]

As discussed (a while ago) in https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Move.20some.20tactics.20to.20separate.20package.3F, the on_goal tactic should move to std. (Immediate motivation is that I need it in tactic scripts generated by Aesop.) I'd be happy to make PRs to mathlib4 and std4 if that helps.